1. Field of the Invention
The present invention relates to an image forming method in which an ink is ejected by an inkjet method to form an image.
2. Description of the Related Art
As an image recording method for recording color images, various methods have been proposed in recent years. In all of the methods, qualities and grades of images or recorded matters are highly required.
Recently, in the case of carrying out inkjet recording, improvement in recording suitability in order to record at a high speed in accordance with a single pass system, which enables recording with a single operation of the head, instead of a shuttle scanning system which has been generally used hitherto, or in order to form images on both sides of a recording medium is expected.
In conventional inkjet methods, since the printing speed is slow, solvents in the ink are dried or penetrate into the recording medium during printing. Therefore, strength of the spotted ink dots has been ensured. Further, since the ink speed is slow in the conventional methods, the conventional inkjet methods have not been used for printing of a large number of sheets of a recording medium, as well as the obtained recording media have never been stacked in a short time.
However, it has been found that, since in the case of forming an image at a high speed utilizing an inkjet method, the time required for drying is short and recording media are successively stacked, solvents in the stacked recording media can hardly vaporize, and as a result, the recording media are to be stacked with the image units being soft and with the strength of the image units being not ensured, which may cause a phenomenon (which is called stacker blocking) in which images transfer to the backside of the recording media printed successively or the recording media cause sticking to each other to destruct images. This phenomenon appears more remarkable as the number of stacked sheets of a recording medium gets greater and as the area of the recording medium gets bigger, and further, the phenomenon appears more remarkable when a recording medium exhibiting a lower penetration speed is used. For example, stacker blocking does not occur in the case of printing at a low speed onto a recording medium into which ink solvents penetrate quickly, as described in Japanese Patent Application Laid-Open (JP-A) No. 2000-235023.
Further, in offset printing, solid particles are generally sprayed immediately after printing, and therefore, stacker blocking hardly occurs.
In connection to the above, in an inkjet printer in which a double-sided printing mechanism is provided and printing is carried out on a prescribed plain paper at a recording speed of 10 inch/s or higher in order to carry out high-speed recording onto plain paper, a method in which an ink composition containing water, a pigment, and a urethane resin emulsion is used is disclosed (see, for example, JP-A No. 2002-235023). Moreover, as an inkjet recording ink which is employed in an inkjet recording method in which heat is applied to an ink, that is ejected from an ink discharge head, at the moment the ink reaches a recording medium, an ink containing a pigment, a low MFT (minimum film-forming temperature) resin emulsion having MFT of from 60° C. to 100° C., and a high TG (glass transition temperature) resin emulsion having TG of from 140° C. to 200° C. is disclosed (see, for example, JP-A No. 8-283636). It is described that, according to this technology, drying proceeds faster (see, for example, JP-A No. 8-283636).